Investigators have expended considerable effort in the study of surfaces in space which are selectively passive to the transmission of electromagnetic energy. Such studies have evolved a broad spectra of designs for a considerable variety of transmission and reception applications. For example, the aerospace industry has a continuing interest in improved radome structures capable of performance under rigorous high-speed all-weather aircraft applications. Conventionally structured radomes formed of rigid dielectric or ceramic materials have evidenced a broad range of operational problems. Precipitation noise encountered at high speed and occasioned by static charge buildup and subsequent discharge to the airframe has represented a hindrance to the performance of enclosed equipment. As requirements for scan angle flexibility have enlarged, a variety of effects are encountered. For instance, a transmission loss and phase distortion are witnessed. Further, the equipment enclosed by more conventional radomes is susceptible to lightning damage as well as to thermal problems developed by poorly controlled frictionally induced skin heating.
Over the somewhat recent past, investigations of scattering from periodic arrays of slots and their performance as band-filters of electromagnetic radiation has suggested the application thereof, inter alia, as metallic radomes. Metallic radomes provide such advantages as the elimination of precipitation noise, inherent lightning protection; improved shielding against spurrious low frequency pulses due to the above-noted band-pass filter characteristics; and a potentially improved mechanical strength for the radomes. However, due to aerodynamic design constraints, the geometric shapes which these radomes must assume, for example ogival or conical, have developed a need to accommodate scan angles of incidence of values of 80.degree. and above. Without correction, scanning over such geometry will engender a lack of pass bandwidth constancy in dependence upon the asserted E- and H-plane angles of incidence.
Developments seeking to cure certain of these deficiencies of the metallic radomes include the utilization of arrays of resonant short dipole elements of length less than one half wave length which are loaded by a slot structured in the manner of a two-wire transmission line, as described in U.S. Pat. No. 3,789,404. As another approach to improving the noted deficiencies, reactively loaded periodic tripole slot elements have been developed as are described in U.S. Pat. No. 3,975,738. However, these design approaches have, for the most part, failed to meet the very rigid criteria of maintaining operational stability even though incident scan angles reach values of 80.degree. and above.
The same design approaches as discussed above for metallic radomes also have been utilized to develop a space filter for use as a low loss dichroic plate permitting a simultaneous single antenna transmission of both X and S band energy. In this regard, mention may be made of U.S. Pat. No. 3,769,623.
A broadened utility for space filters of the type described requires performance over a relatively broadened band pass region of interest with lowered transmission loss within that region. Such performance is characterized by transmission curves which exhibit a flatness in the region of interest at unity transmission coefficient and relatively sharp skirts. To the present, such performance has represented an elusive goal, particularly where the noted relatively high scan angles of incidence are contemplated.
Those artskilled in the investigation of scattering from periodic arrays will recognize that the theory applied in connection with periodic arrays of recurring slots are directly applicable to periodic arrays of dipoles. In terms of circuit concepts, periodic arrays of dipoles are band-stop, or reflection filters. Within their operating band, properly designed arrays of dipoles reflect incident signals in a manner comparable to a highly-conductive solid metal surface. Outside of this reflection, however, incident signals pass through the array dipoles. Periodic arrays of slots, on the other hand, perform a complementary role, with respect to dipole arrays. Slot arrays function as an electromagnetic window within their operating bands or pass-bands permitting incident electromagnetic signals to pass through the array. Outside of this operating band the array becomes opaque, reflecting the incident signal.